The present invention generally relates to anti-skid control and more particularly, to an antiskid control device which is provided with a means for properly estimating a speed of a vehicle body of a motor vehicle.
An antiskid control device is arranged to control a slip ratio of wheels to a value falling within a range enabling most efficient achievement of a frictional force exerted between a road surface and tires. It is to be noted that the slip ratio of the wheels is expressed by [(vehicle speed)-(wheel speed)]/(vehicle speed).times.100 (%). Thus, the vehicle speed is an important factor in antiskid control. However, in most antiskid control devices in practical use at present, a means for directly measuring the vehicle speed is not provided. The vehicle speed is estimated from the wheel speed, and more specifically the wheel speed is calculated by using the higher wheel speed of the undriven wheels.
During normal operation of the motor vehicle, the higher wheel speed of the undriven wheels (free from wheel spin) can be regarded as the vehicle speed itself. It is to be noted that the term "wheel spin" in this specification represents a phenomenon in which tires rotate at an extremely high speed relative to a road surface because a driving force transmitted from an engine to the tires has exceeded a frictional limit between the tires and the road surface. However, during braking (especially, during execution of antiskid control), when each wheel randomly either approaches towards locking or recovers from locking, it is impossible to obtain an accurate vehicle speed from the wheel speed of a single wheel, even if the wheel is an undriven wheel.
Thus, in this case, it is desirable that the highest wheel speed at each time is selected from the wheel speeds of as many wheels as possible and is subjected to filtering, etc. to obtain the vehicle speed.
To this end, such a method may be adopted in which the vehicle speed is estimated from the highest wheel speed of the undriven wheels during nonexecution of antiskid control and from the highest wheel speed of all the wheels during execution of antiskid control. However, when the motor vehicle runs on a rather bumpy road surface, a case arises that a change of wheel speed due to the bumps is erroneously interspreted as a locking of the wheels. Namely, if the motor vehicle runs on the bumpy road surface in a state where wheel spin of the driven wheels may take place, antiskid control may be started. At this time, if the estimated vehicle speed is calculated from the driven wheels being subjected to wheel spin, the vehicle speed becomes extraordinarily high. Hence, it is decided that each wheel has a large slip ratio, and a pressure reducing command is issued to a brake system. Thus, braking is not effected even if the driver kicks the brake pedal fully.
This is a quite dangerous state for the motor vehicle. In order to obviate such an undesirable phenomenon, the following conntermeasure may be taken. A detection means for detecting the activation of the brake pedal is provided. During execution of antiskid control, the estimated vehicle speed is calculated from the highest wheel speed of all the wheels (including the driven wheels) only at the time when a brake pedal signal indicates that the brake pedal is activated. In this countermeasure, the estimated vehicle speed is calculated from the highest wheel speed of the undriven wheels when the brake pedal signal indicates that the brake pedal is not activated, i.e., when an accelerator pedal may be activated resulting in wheel spin of the driven wheels.
However, in this countermeasure, when the brake pedal detection means malfunctions or in the case of a front-wheel drive type motor vehicle with a parking brake provided at its rear wheels, the parking brake is activated prior to activation of the brake pedal, attention is paid only to the undriven wheels. Hence, the accuracy of the estimated vehicle speed during random approach of the wheels towards locking drops and, in an extreme case, although the vehicle body is being actually displaced, the estimated vehicle speed assumes zero, Thus, antiskid control is not performed.